The failure to correctly duplicate and segregate chromosomes during cell division is a common instigator of neoplasia. In the baker's yeast S. cerevisiae, defects in chromosome stability and DNA repair are associated with mutations in the SMC5 gene. Found in many organisms including humans, Smc5 is a central member of the Smc5/6 protein complexes. The goal of this proposal is to understand how the Smc5/6 complexes maintain the integrity of the genome. This will be accomplished using parallel genetic, molecular biological and biochemical approaches. Yeast with mutant alleles of the SMC5 gene will be examined for their ability to repair DNA, their cell-cycle checkpoint activation, and their DNA replication competence. Synthetic-lethality and high-copy suppression genetic screens will be performed to place Smc5 in the network of proteins controlling the cell cycle and chromosome maintenance. The location of the Smc5/6 complexes will be probed using chromatin immunopreciptation. The complexes will be biochemically purified and ultrastructure visualized by electron microscopy. To understand how the purified Smc5/6 complexes interact with DNA, they will be assayed for DNA binding/bending and ATP hydrolysis activities.